The present invention relates to strain or stress sensors which include a first planar conductive element, a second planar conductive element, and a flexible graphite sheet sandwiched between the first and second planar conductive elements. The flexible graphite sheet includes a mass of expanded graphite particles compressed together in the absence of a binder. Prior to compression, the expanded graphite particles have a c direction dimension which is at least 80 times that of the graphite particles from which the expanded particles are formed. The sensor is configured so that compression of the sensor causes a change in resistance which can be correlated to strain or stress. The present invention also relates to a strain or stress sensor which includes a polymeric composite, a first electrically conductive lead connected to a first point on or in the polymeric composite, and a second electrically conductive lead connected to a second point on or in the polymeric composite. The polymeric composite includes a polymeric matrix material and carbon filaments having diameters of from about 0.01 .mu.m to about 1.0 .mu.m dispersed therein. Methods for detecting strain or stress are also disclosed.